Though the problem of authorship of the Pastoral Epistles has been debated since the 18th century (and perhaps a little before), P.N. Harrison launched the attack against Pauline authorship in the modern era. His studies of word frequency in the Pastorals are exhaustive.

Harrison's thesis is the Fragmentary Hypothesis. He thinks there are some bits of "genuine" Paul in the Pastorals, just not much. He ascribes their authorship to what he calls a "dedicated Paulinist" who had access to Paul's letters, plus some heretofore unknown fragments, that he merged into three different pseudepigraphal letters.

Harrison is worth reading simply because he states his point so well. However, one must realize that (on my reading of it) Harrison isn't writing to explore an idea, he's writing to prove a point. The polemic is tough to get through at times, but keep at it. It's worth the read.